The firmament in the OT and later Jewish tradition is a solid structure supported by the earth that contains the visible heavens. The sky has an appearance of a dome extending from the horizons; it is in the expanse of this dome that the sun shines, the moon and stars move, and the birds fly. Because this notion is inconsistent with modern cosmology, the Society insists that the Hebrew word rqy` only means "expanse" and not something solid. The basis for this is the fact that the Priestly narrative in Genesis 1 construes the luminaries as placed within the rqy` (v. 14), which is the same place where birds fly (v. 20), but in fact the empty space is simply what is contained within the dome or vault -- just as one could place a fish in a bowl. The term comes from a root meaning "to beat out with a hammer," which clearly implies a solid object. Job 37:18 thus refers to God as "beating out (trqy`) the heavens hard (chzqyn) as a mirror of cast bronze" when he created the heavens. The heavens are probably compared to a mirror because they appear to shine forth their own light; they are frequently described as brilliant, luminous, and sparkling, e.g. "The learned will shine as brightly as the firmament of heaven (yzhrw k-zhr h-rqy`)" (Daniel 12:3), "The shining firmament of heaven (stereóma kathariotétos ouranou) appears in a glorious spectacle as the splendor of the heights" (Sirach 43:1). The heavens are also described as stretched out over the earth and upon which God's palace is built above:
Psalm 104:2-3: "He wraps himself in light as with a garment; he stretches out the heavens like a tent and lays the beams of his upper chambers (h-mqrh `lywtyw) on their waters (b-mym). He makes the clouds his chariot and rides on the wings of the wind".
Ezekiel 1:22, 26: "Spread out above the heads of the living creatures was what looked like a firmament (rqy`), sparkling like ice (k-`yn h-qrch) and awesome... Above the firmament (mm`l l-rqy`) over their heads was what looked like a throne of sapphire, and high above on the throne was a figure like that of a man".
4Q403 1:41-44: "Sing to God, awesome in power, all you wise spirits of light, to exalt together the utterly brilliant firmament of heaven (rqy` zw twhr thwrym) that girds his holy temple. Praise him, divine spirits, praising forever and ever the firmament of the uttermost heaven (rqy` rwsh mrwmym), all its beams (qyrwtw) and walls, the work of its construction".
The upper chambers have their beams and walls constructed on the firmament which is stretched like a tent over the earth, and the heavenly palace is built "on their waters," compare Psalm 29:2, 10: "The voice of Yahweh is over the waters, the God of glory thunders, Yahweh thunders over the mighty waters .... Yahweh sits enthroned over the flood, Yahweh is enthroned as king forever". This composition is extremely close to Canaanite poems referring to Baal and El. In the case of the latter, El rules from his own holy mountain (Mount Hermon, the source of the Jordan River) which lies at the meeting-place of the heavenly deep and the subterranean deep under the mountain. This dwelling place is a garden from which the rivers flow that irrigate all creation:
KTU 1.4 iv 20-24: "[Athirat] set her face towards El at the source of the rivers, at the midst of the springs of the two deeps. She rolled back the Tent of El, and came into the pavilion of the King, the Father of the Bright One".
KTU 1.100 R 3-4, V 60-64: "Shapsh, my mother, carry my voice to El at the source of the rivers, at the confluence of the two deeps (mbk nhrm b'dt thmtm)...[Horon] set his face towards the Tigris abounding in rain, and well-watered Tigris, he uprooted from among the trees the tamarisk, and from among the shrubs the tree of death".
Psalm 42:4-7: "I am on my way to the wonderful Tent, to the House of God, among cries of joy and praise and an exultant throng. Why so downcast, my soul, why do you sigh within me? Put your hope in God, I shall praise him yet, my savior, my God. When my soul is downcast within me, I think of you; from the land of Jordan and in Hermon, in Mount Misar, deep is calling to deep (thwm-'l-thwm qwr') as your cataracts roar".
Ezekiel 28:2, 13-14: "You say, 'I am El ('l) in the dwelling of the gods ('lhym); I dwell in the midst of the seas (ymym)'...You were in Eden in the garden of God ... you were on the holy mountain of the gods ('lhym)".
As for Baal, he built his own palace on his mountain (Mount Zaphon) following his victory over the sea dragon Yamm, which was similarly built on top of the deep which he used to provide the rains that water the earth:
KTU 1.101 R 1-9: "Baal sits like the base of a mountain, Hadad settles on the ocean, in the midst of his divine mountain, Zaphon, in the midst of the mountain of victory. Seven lightning-flashes, eight bundles of thunder, a tree-of-lightning in his right hand. His head is magnificent, his brow is dew-drenched, his feet are eloquent in his wrath. His horn is exalted, his head is in the snows of heaven, with the god there is abounding water" (cf. also the sevenfold thunder being the "voice of Yahweh" in Psalm 29).
KTU 1.4 vii 25-39: "Let a window ('urbt) be opened (yptch) in the house, a casement in the midst of the palace! Baal opened a rift in the clouds; his holy voice (i.e. thunder) Baal gave forth; Baal repeated the issue of his lips".
Compare also the motif of the cosmic mountain in post-exilic writings regarding Mount Zion as the source of "living water" giving life to all (cf. Psalm 87:1-7; Ezekiel 47:1-22; Joel 4:18; Zechariah 14:8, 1 Enoch 26:1-2, Revelation 20-22), and in later Jewish tradition, the foundation for the Temple was believed to have kept the subterranean floods at bay; it was said to be the stone on which the world is based (Yoma 54b). What is quite interesting is that the same words used in the Baal Cycle to refer to the "opening" of a "window" in Baal's palace occurs in the OT (cf. Hebrew 'rbwt "window" and ptch "open") to refer to Yahweh opening up the windows of heaven to bring the waters to the earth (Genesis 7:11, 8:2, 2 Kings 7:2, 19, Isaiah 24:18, Malachi 3:10). The mabbul "flood" that Yahweh is enthroned upon in Psalm 29 is also the same word that occurs in Genesis 7-8 to refer to the floodwaters cast down to the earth from heaven.
Although the text is somewhat difficult, the myth of the Birth of the Gracious and Beautiful Gods gives the Canaanite perspective of the vault of heaven as comprised of two gods Shahar and Shalem -- one responsible for dawn and the other for dusk:
"The gracious gods delimit the day, sons of a single day, who suck the nipples of the breasts of the lady, one lip to the earth and the other lip to the heavens. Into their mouths enter the birds of the heavens and the fish of the sea when they stand from delimitation to delimitation" (KTU 1.23 I 62-70).
So here apparently we have two gods who together mark out the day, one bringing forth the dawn and the other bringing the day to an end. They are construed as standing at the edges of the world and rising up from the horizon to the zenith of heaven -- into whose mouths the birds fly at the height of heaven and the fish swim at the bottom. This poetic description is reminiscent of the vault of heaven, with the horizontal horizon in the east or west forming the bottom lip and the upper lip arching up to heaven -- resulting in open mouths in the east and west. This Shahar/Shalem mythology is not representative of the cosmology of the Hebrews, although there are some possible traces in the OT (cf. Genesis 49:24-25, Joshua 13:19, 1 Chronicles 7:10, Psalm 108:2, 110:3-4, 139:9, Isaiah 14:12-13).